1. Field of the Invention
This invention relates in general to spread spectrum receivers and in particular to GPS navigation systems such as those used in terrestrial navigation for cars, trucks and other land vehicles.
2. Description of the Prior Art
Car navigation is conventionally performed using highway and street maps aided, to some degree, by distance measurements from external sensors such as odometers. Improvements over the last 10 years in Global Positioning System, or GPS, satellite navigation receivers has spawned several GPS car navigation systems.
Conventional GPS car navigation systems use the last known position of the vehicle, and the destination data, to compute a route data base, including route and turning data derived from a pre-existing map data base. GPS receivers are conventionally operated with a minimum of 3 or 4 satellites distributed across the visible sky in order to determine, or at least estimate, the four necessary unknowns including xuser, yuser and zuser which provide three orthogonal coordinates to locate the user as well as tuser which provides the required satellite time.
Techniques such as time or clock hold and altitude hold, in which the unknown time or altitude is assumed to remain predictable from a previously determined value, e.g. zest and/or test, have permitted operation of GPS receivers with less than 4 satellites in view. In particular, terrestrial GPS receivers have been operated with as few as 2 satellites to provide a 2 dimensional position solution using both clock and altitude hold.
Because continuous reception from 4 GPS satellites is often difficult to maintain in a car navigation environment, and known clock and altitude hold techniques can only permit operation with at least 2 satellites, known conventional car navigation systems have typically augmented the GPS position information with information from external sensors to provide dead reckoning information. The dead reckoning information is often provided by an inertial navigation system such as a gyroscope.
Augmenting GPS data with inertial navigation data has permitted the use of GPS car navigation even when less than 4 satellites are visible, such as in tunnels and in urban situations between tall buildings. However, the resultant increased complexity and costs for such combined systems have limited their acceptance.
Conventional GPS receivers use separate tracking channels for each satellite being tracked. Each tracking channel may be configured from separate hardware components, or by time division multiplexing of the hardware of a single tracking channel, for use with a plurality of satellites. In each tracking channel, the received signals are separately Doppler shifted to compensate for the relative motion of each satellite and then correlated with a locally generated, satellite specific code.
What is needed is an improved spread spectrum receiver, such as one for use with GPS navigation systems, which avoids the limitations of conventional designs and provides improved results in a wide range of reception conditions.
In one aspect, the present invention provides an improved terrestrial navigation system using a GPS receiver which can continue to navigate with continuous GPS data from less than the 3 or 4 GPS satellites commonly required. The GPS data is augmented with data from another source. The source of the augmentation data may include data from external sensors, data bases including map data bases, and/or knowledge of the physical environment within which the vehicle is to be navigated. The use of such augmentation data permits GPS satellite navigation solutions for stand-alone GPS systems as well as for GPS systems integrated with external sensors and/or map databases with less than 3 or 4 continuously visible GPS satellites.
In another aspect, the present invention provides a GPS receiver in which map data used to determine routing is also used as a source of data augmentation for a single satellite solution by providing direction of travel information.
In still another aspect, the present invention provides a method of augmenting GPS data using information from the physical environment. For example, vehicles are usually constrained to tracks no wider than the width of the roadwayxe2x80x94and often to tracks only half the width of the roadwayxe2x80x94and trains are constrained to the width of their tracks. This cross track constraint data may be used to provide augmentation data and allow the vehicle to continue to navigate with only a single satellite in view. The cross track constraint data permits the computation of along track data useful for calculating total distance traveled to provide a GPS based odometer measurement.
The present invention permits the computation of distance along track for use as an odometer reading while tracking only one satellite. Cross track hold provides along-track data directly which, in the case of a vehicle, directly provides distance traveled information useful in lieu of a conventional odometer reading.
In addition to clock and altitude hold, the present invention uses a technique which may be called cross-track hold in which the single satellite in view is used for determining the progress of a vehicle such as a car along its predicted track, such as a roadway. The data conventionally required from a second satellite is orthogonal to the track and therefor represents the appropriate width of the roadway. This value may be assumed and or constrained to a sufficiently small value to permit an estimate of the value, e.g. yest to provide a mode described herein as cross-track hold while obtaining useful GPS navigation from a single satellite in view.
In other words, in accordance with the present invention, single satellite navigation may be achieved by using the data from the single satellite for on-track navigation information while holding or estimating the time, altitude and/or cross-track navigation data.
The required augmentation data may additionally, or alternatively, be derived from other sources in the physical environment, such as turns made by the vehicle during on-track travel. In accordance with another aspect of the present invention, the vehicle may detect turns made during travel and update the current position of the vehicle at the turn in accordance with the timing of the turn. Turn detection may be accomplished by monitoring changes in the vehicle vector velocity derived from changes in the GPS derived position information or by monitoring changes in the compass heading or by any other convenient means.
In another aspect, the present invention provides a GPS system for navigating a vehicle along a track, including means for tracking at least one GPS satellite to provide on-track information related to progress of the vehicle along a selected track, means for providing an estimate of cross track information related to motion of the vehicle perpendicular to the track, and means for providing vehicle navigation data, such as vehicle position or vehicle velocity, from the on-track information and the cross-track estimate.
In still another aspect, the present invention provides a method of deriving position information from a single GPS satellite by tracking at least one GPS satellite to provide ontrack information related to progress of the vehicle along a selected track, providing an estimate of cross track information related to motion of the vehicle perpendicular to the track, and determining the position of the vehicle from the on-track and the cross-track estimates.
In still another aspect, the present invention provides a method of updating GPS position information for a vehicle navigating on roadways by deriving an indication that the vehicle has made a turn at a particular point along a predetermined track, comparing the turn indication with stored navigation data to select data related to one or more predicted turns at or near the particular point, comparing the turn indication with the predicted turn data to verify that the indicated turn corresponds to the predicted turn, and updating GPS position information to indicate that the vehicle was at the predicted turn location at a time corresponding to the turn indication.
In still another aspect, the present invention provides a GPS system for navigating a vehicle, the system including means for tracking at least one GPS satellite to provide on-track information related to the direction of travel of the vehicle along a selected track, and means for deriving vehicle navigation data from changes in the direction of travel of the vehicle along the selected track.
In another aspect, the present invention provides a method of determining position information from GPS satellites by selecting a route to be traveled, tracking one or more GPS satellites with a GPS receiver mounted for motion along the route; and deriving route specific along track, cross track, altitude and satellite time information from the one or more GPS satellites being tracked.
In yet another aspect, the present invention provides a method of determining position information from GPS satellites by tracking at least two GPS satellites to derive navigation information related to a vehicle in motion along a track, estimating cross track hold information related to possible motion of the vehicle transverse to the track, determining along track position information related to the position of the vehicle along the track by combining the estimated cross track and the navigation information.
In another aspect, the present invention provides a method of determining position information from GPS satellites by estimating cross track hold information related to motion of a vehicle transverse to a track, tracking two GPS satellites to derive navigation information therefrom, combining the estimated cross hold and navigation information to derive along track information related to the position of the vehicle along the track, selectively estimating clock hold information related to satellite time, and combining the estimated clock hold and navigation information to derive updated cross track hold information.
In a still further aspect, the present invention provides a GPS receiver including means for tracking two GPS satellites to derive navigation information therefrom related to the position of a vehicle, cross track hold means for estimating the deviation of the position of the receiver from a track along which the vehicle-is predicted to be in motion, and data processing means for combining the navigation information from the two GPS satellites being tracked and the cross track deviation estimate to derive along track navigation information related to the motion of the vehicle along the track.
In a still further aspect, the present invention provides a method for operating a GPS receiver under adverse satellite signal reception conditions by creating and using a cross track estimate when only two satellites with adequate geometric coverage are available and using a fast reacquisition mode to update an along track estimate during brief periods of availability of any additional satellite.